1. Field of the Invention
This invention relates to a method for reducing the permeability of the more permeable zones of a subterranean reservoir having nonuniform permeability, and more particularly, to such a method wherein the elevated temperature encountered in the reservoir causes permeability reduction to occur after an injected composition or compositions have passed radially into the reservoir only a short distance from the well so that continuous permeability reduction is effected from nearby the well extending into a more permeable zone a substantial distance.
2. Description of the Prior Art
It has long been recognized that when fluids are passed through a subterranean reservoir having heterogeneous permeability, for example during an enhanced recovery process or during production of fluids from a well, a relatively higher volume of fluids tends to pass through the more permeable strata with little or no fluid passing through the less permeable strata.
It is known to attempt to even out the flow of fluids through such reservoirs, for example, to increase the proportion of the fluids flowing through the less permeable portions of the reservoir, by injecting into the reservoir a fluid which predominantly enters the more permeable strata where it subsequently thickens and/or at least partially sets to reduce the permeability of these more permeable zones and in some cases to plug them completely. As a result, a higher proportion of any fluid subsequently passed through the reservoir will flow through the portions of the reservoir which originally were less permeable.
Among the more recent fluids which have been used to reduce permeability or plug more permeable strata in a reservoir are aqueous solutions or dispersions of a polymer containing a cross-linking agent. For example, U.S. Pat. Nos. 4,009,755 and 4,069,869 to Sandiford each deals with forming plugs in wells wherein there are injected various combinations of: (a) an aqueous solution of a water-soluble polymer, such as polyacrylamide, (b) a cross-linking material that reacts with the polymer to form a polymer-containing plug, such as a compound of a multivalent metal and a reducing agent, a low molecular weight water-soluble aldehyde, or a colloidal hydroxide of a multivalent cation, (c) an aqueous solution of an alkali metal silicate and (d) a gelling agent that reacts with the silicate to form a silicate-containing plug, for example, acid and acid-forming compounds, water-soluble ammonium salts, lower aldehydes and aluminates. In the compositions described in each of these references, a large amount of alkali metal silicate is present, and this material becomes a part of the plug.
U.S. Pat. No. 3,687,200 to Routson describes a method for controlling the flow of aqueous fluids in a porous subterranean formation using an aqueous colloidal dispersion of a water-soluble inorganic compound, such as aluminum hydroxide, prepared in an aqueous solution of a synthetic organic polymeric polyelectrolyte, for example, a polyacrylamide, without a cross-linking agent. In one method of preparing the dispersion, a small amount of an alkali metal hydroxide or an alkaline buffer is dissolved in the polymer solution. Thereafter, a dilute solution of a suitable metallic salt, such as a soluble salt of aluminum, is added to the polymer solution to precipitate aluminum hydroxide in the form of a stable colloidal dispersion of the insoluble aluminum hydroxide. The resulting composition causes a high pressure drop when forced through a permeable core. An alternative method of preparing the dispersion involves addition of a precipitant such as sodium sulfide to the composition instead of the alkali metal hydroxide. In yet another method applicable in certain cases, the composition containing the colloidal dispersion is generated inside a subterranean formation. Specifically, if the connate water contains soluble sulfide ions, a solution of a metallic salt in an aqueous solution of polymer is injected to form the colloidal insoluble metallic sulfide in the interstices of the formation. Otherwise, the dispersion plugging agent is produced before the composition is injected into the formation, thereby promoting plugging at the well sidewall and in the immediate vicinity of the well rather than in the formation a substantial distance from the well.
U.S. Pat. No. 3,841,399 to Ryan describes forming in situ a polymer-thickened aqueous waterflooding composition by injecting into a formation a low viscosity, potentially hydrolyzable, synthetic aqueous emulsion polymer, such as a water dispersion of acrylonitrile copolymerized with ethyl acrylate, and an aqueous solution of sodium hydroxide. The polymer is eventually hydrolyzed in the subterranean formation to form a high viscosity composition. The caustic solution can be added in admixture with the polymer compositions or separately following the polymer composition. No cross-linking agent is employed.
U.S. Pat. No. 3,825,067 to Vestal describes a method for dissolving a polyacrylamide polymer in water to form a composition useful as a secondary recovery drive fluid. The powdered polymer is mixed with dilution water, the pH is adjusted to between 7 and 14 by the addition of a base such as sodium hydroxide or trisodium phosphate, and the resulting solution is heated to simultaneously hydrolyze and solubilize the polymer. No cross-linking agent is utilized. The composition does not form a plug in the reservoir or otherwise alter reservoir permeability, but rather functions as a drive fluid to displace oil from the reservoir.
While each of the aforementioned treatments has met with some success in particular applications, the need exists for a further improved well treating process to selectively reduce the permeability of the more permeable strata of a reservoir having heterogeneous permeability, especially in the immediate vicinity of a well penetrating such strata and extending continuously into the formation as far as is desired. Moreover the prior art processes often encounter practical difficulties. For example, in those processes employing a polymer and cross-linking agent, if the two materials are mixed together in an aqueous solution at the surface, they tend to initiate reaction immediately. By the time the resulting composition is pumped down a well and into a reservoir, the composition often has already begun to increase in viscosity and may be difficult to force into even the higher permeability strata. The same is true to a lesser extent if separate aqueous solutions of polymer and cross-linking agents are simultaneously pumped down a well and mix as they pass through the tubing. On the other hand, if one of the compositions is pumped first followed by the second, there is less chance of reaction occurring until after the compositions have entered the reservoir. However, mixing in a reservoir of separately injected compositions is difficult to control. And if the composition is premixed but contains an alkaline ingredient intended to delay the cross-linking reaction until the pH is lowered by contact of the fluid with the reservoir, the plug does not form until the fluid has passed some distance from the well into the reservoir. What is desired, therefore, is a composition capable of reducing permeability that can be injected down a well and into a reservoir as a low viscosity, easily-pumped liquid and which reacts after injection at a rate highly accelerated by the elevated temperature encountered within the formation of the reservoir to form a viscous, permeability-reducing composition or plug in the immediate vicinity of the wellbore, preferably the temperature-activated composition being used to form a continuous gelatinous plug extending from nearby the well as far into the reservoir as is desired.
Accordingly, it is a principal object of the invention to provide a method for reducing the permeability to fluids of the more permeable strata of a reservoir having heterogeneous permeability in the immediate vicinity of the wellbore.
It is a further object of this invention to provide a method wherein the permeability reduction occurs nearby or in the immediate vicinity of a well penetrating such reservoir and extends into the reservoir as far as is desired.
It is a still further object of this invention to provide such a method wherein the permeability reduction is achieved near to the well by injecting into the reservoir via a well a relatively low viscosity aqueous base composition which reacts at reservoir temperature to form a relatively high viscosity composition and/or plug after passing into the reservoir.
An additional object of this invention is to provide a method of continuous injection whereby a substantially continuous plug can be formed between two impermeable strata extending from nearby the well into the reservoir as far as may be desired.
Another object of this invention is to provide such a method wherein the point of the onset of permeability reduction and the extent of plug formation in a reservoir can be controlled.
Other objects, advantages and features of the invention will be apparent from the following description and claims.